Tapering bricks for the dome

[Novices first build]

Hi All

I've been reading these forums for a while now with keen interest so just before lockdown 2020 I decided to take the plunge and make an oven which still hasn't reached completion for one reason or another

I'm at the stage where I've built my foundation slab, oven stand and concrete slab to hold the entire weight of the oven.

So far I've managed to cut two layers of cal sil board below two layers of 64mm brick that would be used as a heat bank for the oven floor followed by a soldier course of bricks spanning the full diameter of the oven on top of which I would start my dome.

However the problem seems to be that I can't seem to find a way to taper the bricks to collapse the v shaped holes at the back alloqimg me to use less cement but at the same time bring the bricks up high enough to match my dome arch..

Any help would be much appreciated

[Sixto]

There are lots of V-shaped spaces between bricks that you could be referring to, so we'll need more information. There's the horizontal V-shaped mortar wedge between rectangular bricks that allows you to have a circular shape.. Then there's the V shaped mortar wedge between each vertical course of bricks which allows you to curve the vertical face of the dome. Finally, there's the inverted V shape that starts to become more prominent 4-or-5 courses above the base, as the bricks both tilt-up on the outside and rotate around the center or your dome, so that their top inside corners touch before the bottom inside corners do.

None of these V-shaped wedges cause a problem per se, if you grout them with one inch of heat-resistant mortar on the inside face, and back them up with regular mortar (to save money) or if you are using homebrew mortar like I did, just fill them with the same thing all the way through. The heat-sink and heat-resistant properties of fire brick and homebrew are very similar, so your dome will perform as if it is a solid unit.

Some people want to eliminate the horizontal wedge betwee the sides of two adjacent bricks, which can be achieved by cutting the sides so the inside rectangular face of brick is narrower than the outside face rectangle. This is labeled as the Angle, on the image below from UtahBeehiver . This is a fairly easy cut to make, turning a rectangular brick into a truncated wedge-shape.


Some people (including me, when I started building) object to the look of the inverted V's on the inside of the dome, which get more prominent as you go higher up the dome, and that can be corrected by making a slanted cut on both sides of the brick (AKA Bevel) thus making them into a slight trapezoid shape instead of a rectangle when viewed from the dome center. See bottom of image above from UtahBeehiver . This is a compound cut, meaning you are angling the brick in two dimensions relative to the saw blade, which increases the difficulty.

If you want to eliminate all mortar wedges and have all mortar joints in the dome be parallel and tight (including the vertical joints between courses) it will require a lot more bricks, and a lot of cutting. I spent 2 months just cutting the Bevels to about 1" depth in order to minimize the look of inverted V's of my bricks. Was it worth it for me? It was when I started, it isn't now. I did not cut any Angles on my bricks, just filled the joints between them with homebrew as I laid each brick.

The angles you'll need to cut depend on the size of your dome, the size of your brick, and the height of the first horizontal course above your soldier bricks. There are two spreadsheets at the following link that can help you figure them out .https://community.fornobravo.com/for...eet-calculator.

All of this will be a lot easier if you have a wet-saw with a 14" blade, adjustable cutting height, and a nice sliding bed with a tilt/angle jig. If you want to do this, it can be done, but I suspect it does not improve the performance of any given oven or the quality of the food it cooks in it in a significant way, it is really something people do to fullfill a personal desire for craftsmanship.

Good luck! and enjoy the process... at about month 10 I started to burn-out and temporarily regretted not having bought an Ooni instead. But after a few days off, I had renewed energy to press on.

[UtahBeehiver]

I am a little uncertain about what you are doing on the floor. Are you placing two different layers of brick on the floor. If so what do you envision how you will use the oven. Typically, ovens used for extended baking of bread have thicker floors (ie greater than say 50-75mm or 2 -3 inches). Thicker floors mean higher thermal mass but if you are going to do pizzas and some secondary baking 50mm is sufficient. Ovens with high thermal mass also take much more fuel to heat up. Insulation if more critical than thermal mass in most oven applications.

Back to the inverted V, as Sixto described it start to occur around the 3-5 course and can be reduced by a bevel cut but you don't have to make the cut the entire length (front to back). Look at JR Pizza's build. Most all questions are answered in the blog you just have to root around for them.

You might want to consider a cast oven as well. Nick JC from the UK did a very nice job on a cast oven and document the process well and suppliers.

[Novices first build]

Hey

Thanks for the reply Sixto.

My internal diameter of the oven is 1316mm (52 inches) and I'm hoping to reach an internal dome height of 520mm (20.5 inches) so it's an ellipse instead of being a gradual arch.

I've already cut the angle (as shown above) onto the bricks to collapse the horizontal V shaped mortar wedge but I'm having trouble working out what shape the brick would need to be to collapse the vertical course which would allow me to curve the vertical face of the dome. A way to work out dimensions would be great.

Ideally I'd like to bevel all the bricks to be able to successfully collapse all the V shapes and get each brick as tight as possible as the pre mixed mortar I'm using can only be spread around 2/5mm thick(I've not been able to find any suitable alternative to the fire cement locally)

Thanks in advance for any suggestions

[Novices first build]

Hi Utahbeehiver

Thanks for the reply earlier.

I've used two 64mm bricks bedded onto one another in a with air setting cement in a herringbone pattern with staggered joints to avoid any heat loss. I was trying to reach 100mm for the floor but the slips were more expensive to buy to reach the thickness I wanted so had to compromise and its now 128mm high.

Ideally I'll be using the oven for pizza and then a day or so later for bread so I've tried to keep it fully insulated below the bricks too by placing 100mm of calsil boards below the firebrick floor to retain the heat for as long as possible. Looking back its possibly over kill however at the time I thought it was the best strategy. Its now 228mm high on the stand it sits on but I'm hoping this will be fine.

I'll take a look at the JRs build and try and upload a few photos on here tomorrow for you guys to see.

Thanks for the help so far

[Sixto]

Ideally I'd like to bevel all the bricks to be able to successfully collapse all the V shapes and get each brick as tight as possible as the pre mixed mortar I'm using can only be spread around 2/5mm thick(I've not been able to find any suitable alternative to the fire cement locally)

I don't know of anyone who has done this, or any reference that you could use to calculate those angles. It would be tricky since as you say you are going for a neapolitan dome with a shallow arch for the given diameter. I believe the Excel spreadsheets assume a hemispherical dome...

The best I can think of is to find someone that can do a CAD drawing of the internal dome profile you want, and then read-off the angles of each brick course required to make the curve you want.

Have you considered mixing your own homebrew refractory mortar with Sand, Portland Cement, Hydrated Lime, and Clay powder in these proportions 3:1:1:1? The beauty of the homebrew recipe is that it's very forgiving, and allowed me to patch gaps later. It is probably not as heat resistive as the premixed mortar you have, but for pizza temps, it's good enough to last the rest of my life expectancy, and many folks on the forum cast their own dome using that recipe. I bet it could save you weeks if not months of frustration cutting and mortaring the very tight joints required by the premix.

[UtahBeehiver]

To reiterate, you do not have to "bevel" the full length of the brick, only the first inch or so from the inner face where the two opposing brick conflict with each other and form the "inverted V" then the wider back side gaps are filled with mortar for structural strength and bonding. So once you figure how much and the angle for each course you cut a half dozen bricks then adjust accordingly. I did full tapers and bevels and have very tight mortar joints but IMHO it is not worth the effort and I would only partial bevels to achieve tight inside the dome mortar joints and back mortar the wider backside joints.

[Pizzarotic]

Ideally I'd like to bevel all the bricks to be able to successfully collapse all the V shapes and get each brick as tight as possible as the pre mixed mortar I'm using can only be spread around 2/5mm thick(I've not been able to find any suitable alternative to the fire cement locally)

As Utahbeehiver has explained, it's only necessary to bevel approximately 1" of the inner face of the brick to achieve minimal mortar joints on the inside of the dome. Generally accepted good practice is to use a slither or wedge of brick to fill the excess spaces on the outside to keep your mortar thickness within spec.

[david s]

Because refractory mortars recommend thin mortar joints, which is not possible if there’s a large wedge towards the outside, then there could be failures if thick joints are used. I think the problem is the risk of steam spalling in the thick mortar in much the same way as castable. This problem can be largely averted by the addition of burnout fibres. Presumably the same would apply to thick homebrew joints. It may therefore be advisable to add some to both mortars if using thick joints. The problem of difficult dispersion requiring more extensive mixing, increases mixing time which is problematic for mortars with limited working times.

[Novices first build]

Hi guys. Thanks for the help so far as well as the replies. Id like to share the photos of progress so far but for some reason it keeps telling me I've exceeded the max size so I'll try doing them in a separate post.

I'll look into the homebrew method as it sounds like a good idea which will save me time doing all the cuts so thank you for the suggestion along with the ratios.
Are you aware of what industry uses the clay? I'm not familiar with the product so wouldn't know where to start looking.

Also Utahbeehiver I've had a look for the Jr's build you mentioned but haven't been able to find it on here so I've no idea if I'm doing something wrong or looking in the wrong place but a link would be great.

[Sixto]

Hi guys. Thanks for the help so far as well as the replies. Id like to share the photos of progress so far but for some reason it keeps telling me I've exceeded the max size so I'll try doing them in a separate post.

I'll look into the homebrew method as it sounds like a good idea which will save me time doing all the cuts so thank you for the suggestion along with the ratios.
Are you aware of what industry uses the clay? I'm not familiar with the product so wouldn't know where to start looking.

Also Utahbeehiver I've had a look for the Jr's build you mentioned but haven't been able to find it on here so I've no idea if I'm doing something wrong or looking in the wrong place but a link would be great.

I use the Windows Resize feature to get my photos small enough to upload.

i got my 50# clay dust bag at my local firebrick supplier, but you should also be able to use any powdered clay available at a ceramics or pottery supply store.

here is a link to JRPizza's build: https://community.fornobravo.com/for...er-build/page3

[UtahBeehiver]

photos limited to abt 1.25mg and abt 5 per post.

[david s]

To resize I email myself the photos and select medium size.